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    Choosing the right solution approach: The crucial role of situationalknowledge in electricity and magnetism

    Elwin R. Savelsbergh*

    Freudenthal Institute for Science and Mathematics Education, Utrecht University,

    P.O. Box 80000, NL-3508 TA Utrecht, The Netherlands

    Ton de JongUniversity of Twente, P.O. Box 217, NL-7500 AE Enschede, The Netherlands

    Monica G. M. Ferguson-Hessler

    Eindhoven University of Technology, P.O. Box 513, NL-5600 MB Eindhoven, The Netherlands(Received 31 March 2010; published 28 March 2011)

    Novice problem solvers are rather sensitive to surface problem features, and they often resort to trial

    and error formula matching rather than identifying an appropriate solution approach. These observations

    have been interpreted to imply that novices structure their knowledge according to surface features rather

    than according to problem type categories. However, it may also be the case that novices do know problem

    types, but cannot map the problem at hand to a known type, because they fail to create a sufficiently well-

    elaborated problem representation. This study aims to distinguish between these explanations. In this

    study novice physics students at high and low levels of proficiency completed two problem-sorting tasks

    from the domain of electricity and magnetism, one with and one without elaboration support. Resultsconfirm that these students do distinguish problem types in accordance with their required solution

    approaches, and that their problem-sorting performance improves with elaboration support. Therefore, it

    was concluded that their major difficulty lies in the process of matching concrete problems to a proper

    category. Within-group analysis revealed that the performance of proficient novices clearly improved with

    elaboration support, whereas the effect for less proficient novices remained inconclusive. The latter

    finding is explained from the less proficient novices problem representations being too fragmented to

    integrate new information. These results suggest that, in order to promote schema-based problem solving,

    instruction in the domain of electricity and magnetism should be based not so much on restructuring the

    conceptual knowledge base but rather on enriching situational knowledge.

    DOI:10.1103/PhysRevSTPER.7.010103 PACS numbers: 01.40.Fk

    I. INTRODUCTION

    It is an often heard complaint that novice problem

    solvers will skip most of the problem analysis and start

    calculating right away. Many instructors strive to teach

    their students more expertlike approaches, [1] such as

    analyzing the problem and devising a solution plan [2],

    but in general such interventions turn out to be little

    effective [3,4].

    The experts behavior has been explained from their

    knowledge being organized in problem schemas, i.e.,

    knowledge structures organized around a problem type

    with an associated solution approach. Once the right prob-lem type has been identified, the expert will also know an

    outline for a solution approach [5]. It seems worthwhile to

    promote the formation of powerful problem schemas

    among physics learners. However, in order to do so effec-

    tively, one needs to know what exactly is missing from the

    novices knowledge structure.

    The classical problem-sorting experiment by Chi,

    Feltovich, and Glaser might be among the most well-known

    studies to discriminate between novice and expert knowl-

    edge structures [6]. In their study, Chi et al. let several

    experts and novices sort a set of mechanics problems. The

    experts would spontaneously sort these problems according

    to their solution approaches (i.e., second law, conservation

    of energy, conservation of momentum), whereas the novi-

    ces would sort according to surface features (angular mo-

    tion, springs, inclined planes). Although such studies

    provide convincing evidence for the quality and structure

    of expert problem schemas, the interpretation of the novice

    results is much less conclusive [7]. For instance, whereas

    Chi et al. [6] concluded that novices did not have their

    knowledge organized in problem type schemas, Cooper

    and Sweller [8] found that novices already start to induce

    problem categories after trying only a few problems.

    *[email protected]

    Published by American Physical Society under the terms of theCreative Commons Attribution 3.0 License. Further distributionof this work must maintain attribution to the author(s) and the

    published articles title, journal citation, and DOI.

    PHYSICAL REVIEW SPECIAL TOPICS - PHYSICS EDUCATION RESEARCH 7, 010103 (2011)

    1554-9178= 11=7(1)=010103(12) 010103-1 2011 American Physical Society

    http://dx.doi.org/10.1103/PhysRevSTPER.7.010103http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/http://dx.doi.org/10.1103/PhysRevSTPER.7.010103
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    Moreover, whereas Chi et al. [6] concluded that novices had

    sufficiently elaborate declarative knowledge about the

    physical configurations of a potential problem, many stud-

    ies indicate that novices form only limited representations

    of the problems they are working on [912]. To complicate

    matters, there is clear evidence for qualitative differences

    between more and less proficient novices [9,1316], so,

    even if they both fail to identify a proper solution approach,

    it still may be for different reasons.It should be noted that studies about problem schemas

    were conducted in different domains. For instance, the

    experimental task of Chi et al. [6] was drawn from the

    domain of mechanics, whereas the studies of de Jong and

    Ferguson-Hessler [9] were conducted in the field of elec-

    tricity and magnetism. Although studies in both fields

    confirm that expert problem solving knowledge tends to

    be organized in a limited number of problem solving

    schemas, it is not self-evident that more specific findings

    about problem schemas generalize across domains. First,

    there may be considerable structural differences between

    domains: in mechanics the deep features of a problem tend

    to be associated with a small number of conservation

    principles (conservation of energy, momentum, angular

    momentum), which can be sharply distinguished from

    superficial features such as spatial symmetries. By con-

    trast, in the field of electricity and magnetism, spatial

    symmetries are crucial to the choice of an appropriate

    solution approach. Moreover, mechanics differs from other

    physics topics, such as electricity and magnetism, in its

    situational features being more concrete, and in involving

    more everyday knowledge [1719]. On the one hand,

    students experiences in the physical environment may

    help them to develop a clear representation of a mechanics

    situation, whereas, on the other hand, their experience-based expectations may also turn out to be misconcep-

    tions leading them in the wrong direction [20,21].

    Therefore, in order to generalize educational implications,

    the experimental findings must be validated across multiple

    domains.

    Educational implications are clearly different depending

    on which explanation holds true: If novices rely on alter-

    native category structures, we may aim to restructure their

    knowledge, by teaching knowledge hierarchies [22], or by

    inducing cognitive conflicts [23,24]. By contrast, if the

    novice categories are created on the spot, building on

    a poor problem representation, instruction should target

    students understanding of the situations and of the situa-

    tional features that make a solution approach into a useful

    one [10,2527].

    The purpose of this study is to find out to what extent

    each of both factors (that is, poor representation of the

    problem situation and missing knowledge of problem type

    schemas) hampers the ability of novices at different levels

    of proficiency to identify solution approaches in the do-

    main of electricity and magnetism.

    A. Identifying a solution approach: Roles of

    elaborations and schemas

    Every problem solving effort must begin with creating a

    representation for the problem, a problem space in which

    the search for a solution can take place [28]. First, upon

    reading a new problem, one forms a text based representa-

    tion [29]. Keywords in the text based representation may

    immediately suggest a solution approach, and both experts

    and novices make productive use of such keywords to

    determine a solution approach [6,3032].

    Starting from the text based representation, information

    has to be selected, knowledge from memory has to be

    added, and the various elements have to be connected to

    form a structured mental representation of the situation

    [29,33]. Even if one has gained a correct understanding of

    the situation, this may not be the optimal representation:

    the same problem may be easy or difficult, depending

    on the way we describe it. For good reason, one of

    Polyas famous problem solving heuristics is, Could

    you restate the problem? Could you restate it still differ-

    ently? [34]. Thus, while for some problems a surface

    representation of the situation may provide sufficient guid-

    ance to find an adequate solution approach, most problems

    will require elaborations in order to identify the deep

    features of the problem.

    Following VanLehn [35], we define an elaboration as

    an assertion that is added to the state without removing

    any of the old assertions or decreasing their potential

    relevance. In many cases, a series of elaborations will

    be necessary before the problem is well evolved. Some

    elaborations will be made on the basis of common sense

    [36], but many will also require domain knowledge, as in

    the following problem:Given two copper cylinders O1and O2, with radii r1and

    r2, the relation between the radii being given byr2 3r1.Both cylinders are centered along the x axis. O2 isgrounded;O1 carries a charge q1 per meter. Compute thepotential at the surface ofO1.

    In the mind of a problem solver, this problem can evoke

    many potentially relevant elaborations:

    The cylinders are concentricO1 is at the inside ofO2the situation is symmetric under rotations aroundthex axisthe situation has translational symmetry alongthe x axiscopper is a conductorin a conductor thepotential is equal everywhereat the surface of a conduc-

    tor, the field is perpendicular to the conductorO2shieldsits inside from external fields and vice versathe potential

    at O2 is zerothe total charge enclosed in a surface thatlies completely in the metal ofO2 amounts to zerothereis a surface charge ofq1per meter at the inside of O2thepotential runs continuously across this surface charge.

    Each elaboration considered by itself provides little

    direction, and hardly any of them could be identified as

    providing the crucial step. However, taken together, these

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    elaborations, also called construction rules, lead to a

    physics representation [5,37].

    In order to build such a representation, one must have a

    sense of the features that matter. As Hestenes notes in com-

    paring physics problem solving to chess playing, the chess

    players attention is automatically confined to the chess-

    board, where the patterns are to be found. But in physics a

    student who hardly knows what the game is about is likely to

    attend to the wrong things [25,38]. Therefore, teaching avocabulary of patterns is a core element of Hestenes

    modeling approach to physics teaching. To a novice, identi-

    fying a solution approach for a physics problem can thus be a

    real insight problem, in the sense that he will find few cues as

    to whether he is close to seeing the approach, until the

    approach has actually been identified [39,40].

    In addition, if the newly inferred information cannot be

    integrated in a coherent problem representation, working

    memory will soon be overloaded with isolated facts. In

    many domains, it has been found that experts have a

    superior recall of problem situations, which indicates that

    they perceive an integrative structure in the situation

    [41,42]. While proficient novices were found to develop

    a coherent mental model of the situation, less proficient

    novices tend to rely on a text based representation of the

    problem, which provides little support to integrate elabo-

    rations [9,14,15]. In line with this finding, there is clear

    evidence that less proficient students are little inclined to

    elaborate on the problems they encounter [9,14,15,43], and

    that their problem solving performance improves if they

    are forced to perform a structured analysis [44].

    The experts knowledge is much richer, based on past

    experience with similar problems. This is reflected in their

    knowledge being structured in problem schemas

    [6,31,45,46]. Such a problem schema would be a coherentbody of all knowledge relevant to a particular basic

    solution approach, which includes relevant theory, proce-

    dures, memories of prototypical problems and features, and

    conditions that must be met in order to make the approach

    useful. If a problem schema is sufficiently rich with proto-

    typical problems and situational features, it will be easily

    matched when studying a new problem. Once the problem

    has been recognized as similar to a known type, the schema

    permits a much more targeted analysis, and furthermore

    directs the steps to be taken for solving the problem. Of

    course, even if one recognizes the appropriate problem

    type and the associated solution approach, one may still

    fail. That is to say, knowing a schema is not a matter of

    black and white. Nevertheless, at least for proficient novi-

    ces, it has been found that the identification of a proper

    schema leads to better problem solving performance [16].

    To sum up, there is clear evidence that experts elaborate

    on a problem to build a rich representation, which then

    easily matches a known schema. Novices build a much

    poorer representation, which does not easily trigger a

    schema. Nevertheless, it may be the case that they do

    know schemas that might be helpful if they could only

    make the right match. Because observations of problem

    solving behavior, and even intervention studies such as

    Dufresneet al. [44], can provide only limited insight into

    participants knowledge structures, various experiments

    have been conducted to assess these knowledge structure

    more directly. In the next section we will review the

    evidence on a most basic element of schema knowledge,

    namely, whether novices do know problem types accordingto their solution methods.

    B. Assessment of schema knowledge: Evaluating

    the evidence

    The most commonly used method to assess subjects

    perceptions of similarities between problems is a sorting

    task [47,48]. In comparison to a full problem solving task,

    problem sorting provides a more direct test of whether

    subjects are able to identify a basic solution approach,

    independent of whether they have the skill to execute the

    chosen approach. There have been alternative approaches

    to assess schema knowledge, such as problem editing [49]and writing down the first step of the solution under time

    pressure [50], but these techniques are more sensitive to the

    quality of the schema content, rather than to the category

    knowledge per se. Interview techniques have also been

    used, but the outcomes of such studies tend to be less

    directly linked to problem solving [21,51].

    Chiet al.[6] used a problem-sorting task to demonstrate

    that novices (i.e., undergraduates who had just completed a

    relevant introductory course) categorize problems accord-

    ing to a qualitatively different structure than experts do. To

    further characterize the difference, they administered a

    second categorization task. In this case they deliberately

    constructed the set of problems such that the similaritiessuggested by the surface features would be at odds with the

    deep structure of the problems. The conclusion of this

    second experiment was that experts sorted according to

    deep structure, whereas the novices attended to the surface

    structure of the problems. With due caution, they conclude

    that although it is conceivable that the categories con-

    structed by novices do not correspond to existing internal

    schemata, but rather represent only problem discrimina-

    tions that are created on the spot during the sorting tasks,

    the persistence of the appearance of similar category labels

    across a variety of tasks gives some credibility to the reality

    of the novices categories even if they are strictly entities

    related [6]. By contrast, in a study among students of

    similar level, de Jong and Ferguson-Hessler [52] found that

    proficient students did sort knowledge elements according

    to problem types, and that only the less proficient students

    tended to sort according to surface features. Hardiman

    et al. [31], using a similarity judgment task, found that

    novices are able to identify the same similarities experts

    see, although they are more easily distracted by salient

    surface features, and they often use the wrong principles.

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    Hardiman et al. also found proficient novices to be more

    likely to base their judgment on physics principles than less

    proficient novices. Similar results were reported by

    Zajchowski [16], based on think-aloud protocols of a prob-

    lem solving task.

    The results of de Jong and Ferguson-Hessler [52],

    Hardiman et al. [31], and Zajchowski [16] suggest that at

    least more proficient novices do know a category structure

    based on problem types. For less proficient students, theevidence is less conclusive. However, even for these stu-

    dents, if they fail to demonstrate their category knowledge

    in a categorization task, it does not necessarily imply that

    they do not have such knowledge. In particular, it should be

    noted that both Chi et al. [6] and Hardiman et al. [31]

    obtained their results with a set of problems where surface

    features and deep structure were crossed. While this is an

    effective approach to demonstrate novices sensitivity to

    surface features, it makes the outcomes less representative

    of what happens in a realistic problem solving setting,

    where a problem solver finds valuable cues with regard

    to the solution approach at all stages of interpreting the

    problem.

    Given these considerations, we expect that, in the do-

    main of electricity and magnetism, novices failure to

    identify proper solution approaches should be attributed

    to a poor problem representation more than to a lack of

    category knowledge. In order to test this claim we will use

    a problem-sorting task where we manipulate the quality of

    the problem representations by providing simple first elab-

    orations. Our central hypothesis is that providing novices

    with this form of elaboration support may lead to a more

    expertlike problem-sorting performance. We also expect

    that the usefulness of the given elaborations will depend on

    the quality of ones problem representation thus far. If onehas a too incoherent problem representation, elaborations

    will remain isolated facts with little added value.

    Therefore, our second hypothesis is that the effect of given

    elaborations will be stronger for more proficient novices.

    II. METHOD

    A. Design

    The hypotheses were tested in a within-subjects design

    where more and less proficient participants completed two

    different problem-sorting tasks, one with and one without

    elaborations. Performance on both tasks was judged by

    comparison to experts problem sorting.

    B. Participants

    Expert participants were three physics faculty who were,

    or had been, instructors in introductory and/or advanced

    electrodynamics courses.

    Novice participants were 80 first-year university physics

    students who had just completed their first introductory

    course on electrodynamics. In The Netherlands, preuniver-

    sity education extends until the age of 18, and over 80% of

    the physics students are 18 or 19 years old at the beginning

    of their first year. The population is rather homogeneous in

    other respects as well: there are 80%90% males, and over

    90% are of Dutch ancestry.Because the population within a single university was

    too small to provide a sufficient number of participants,

    participants were recruited from two different universities

    (hereafter University A and University B). In the

    Dutch educational system, there are no major status dif-ferences between the diplomas of different universities,

    and practical factors, such as vicinity, are the dominant

    factors in determining students choice of university. Both

    student populations and curriculum are similar enough

    across both universities to regard them as samples from a

    single population.

    To make sure that the students had spent some time on

    the topic, the experiment was conducted after the final testhad been taken. First-year students were randomly selected

    from the facultys phone directory and approached by

    telephone until the desired number of participants was

    reached. Participants were paid for their participation.

    TABLE I. Mean test scores of more and less proficient groups. Note that all test scores are on a

    scale of 1 to 10, higher scores are better, 6 is the threshold between pass and fail.

    Test scores

    National exam University tests A University tests B

    More proficient students

    University A M(SD) 8.93 (.59) 7.13 (1.50)

    n 15 15University B M(SD) 8.90 (.66) 7.87 (1.15)

    n 20a 21Less proficient students

    University A M(SD) 7.25 (.59) 4.58 (1.07)n 22 22

    University B M(SD) 7.27 (.43) 5.92 (.88)n 13 13

    aOne participant got admittance on a foreign diploma.

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    In order to classify the participants as less or more

    proficient, test scores were collected both from the national

    high school final examination in physics and mathematics

    and from their previous university tests for classical me-

    chanics, relativistic mechanics, electricity and magnetism,

    calculus and algebra. Because university test scores are not

    directly comparable across different institutions, each uni-

    versity had its own performance subscale (University A: 6items, Cronbach 0:92; University B: 9 items,Cronbach 0:93). The scores on the national highschool examination could be used to rescale the mean

    and variance of the university test scores in order to com-

    pute an overall student ranking.

    Out of the 80 participants, 9 had either omitted a card or

    had mentioned a card more than once on one of their

    problem-sorting forms. As a consequence, the number of

    valid observations for the elaboration effect is N 71. Amedian split on the overall student ranking was used to

    create sufficiently different groups of more and less profi-

    cient participants, with 36 students remaining in the more

    proficient group and 35 students in the less proficient

    group. As shown in TableI, the grade level on the univer-

    sity test scores differs by about two points on a scale of 10

    between the two groups. The less proficient group on

    average scored below threshold on the university tests,

    whereas the more proficient group scored well above. A

    chi-square test confirmed that the distribution of students

    from the two universities over proficiency groups was not

    significantly skewed,21; N 71 3:19,p 0:74.

    C. Materials

    Two sets of 20 problems each were developed. One set

    consisted of problems from the field of electricity, the other

    of problems on magnetism. Our aim was to construct both

    sets in such a way that there would be four essentially

    different solution types in each set. Problems that required

    a combination of multiple approaches were not included.We did not include catch problems of types the students

    would never encounter in their practice problems, and we

    did not manipulate surface features to suggest a systemati-

    cally different ordering. Within these constraints, we took

    care that the solution type could not be inferred from the

    topical area alone. The design procedure started from a

    larger set of problems that was presented to several teach-

    ing staff. Problems that were classified inconsistently were

    removed from the set. After this procedure, a set of 23

    electricity problems and a set of 22 magnetism problems

    were left. Finally, in order to reduce the problems to two

    sets of 20 problems each, and to validate our intended

    categories, we had the three expert participants sort both

    sets, after which we kept the problems on which agreement

    was best. The design of the final sets is presented in

    TableII.

    In order to test for the effect of elaborations, two ver-

    sions were needed for each problem set, one with and one

    without elaboration. The elaborations were designed to be

    close to the original problem statement and to avoid key-

    word patterns that could promote a keyword matching

    TABLE II. Distribution of the problems over topics according to the experimenters.

    Set 1: Electricity Number of problems Set 2: Magnetism Number of problems

    Gauss law 6 Amperes law 6

    Image charges 5 Dipole approximation 5

    Dipole approximation 5 Induction 5

    Coulombs law or superposition 4 Biot-Savarts law 4

    TABLE III. Examples of electricity problems from two different problem categories. Note that for each problem there was an

    elaborated version and a nonelaborated version. The nonelaborated version only gave the normal printed text, the elaborated version

    also gave the italicized text.

    Gauss law problems Coulombs law with superposition

    E10 Compute the field between two concentric spherical shells

    with the inner shell carrying a uniform charge Q1and the outer

    shellQ2

    E5 A point charge q is at the origin and a second point

    chargeq is at a distance ~R from the origin. Compute the

    net electric field at a point ~rj~rj j~RjThe field inside a uniformly charged spherical shell does not

    depend on the charge on the shell

    The net electric field is the sum of all contributions

    E17 Compute the field of a planar charge distribution that

    extends to infinity

    E9 Compute the field at the axis of a charged ring. Charge Q,

    radius rThe field of an infinitely large planar charge distribution has a

    field component perpendicular to the plane only

    At the axis of a charged ring, only the field component along

    the axis remains

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    strategy. Examples of problem cards with elaborations are

    presented in TableIII(for full collection of problem cards,

    see the Appendix).

    D. Procedure

    Participants were to sort both sets of problems with

    each problem presented on a problem card. Half of the

    novice participants received elaboration support on the

    electricity problems, the other half on the magnetism prob-

    lems. The design was counterbalanced to cancel out effects

    of order and task version (TableIV).

    Before doing the first set, participants read the instruc-tions. Unlike in the study by Chi et al.[6], our participants

    were explicitly instructed that problems were to be sorted

    according to solution approach. This was further illustrated

    using the example of how a cook might answer when asked

    about similarities between several dishes. The explicit

    instruction was included because our goal was to find out

    whether participants are able to categorize problems ac-

    cording to solution approach, not whether they would do so

    under all circumstances. The instruction went on with the

    request to read all problem cards in the first set, prior to

    doing any sorting. When the sorting was done, the partici-

    pant would assign a name to each category.

    The first set took approximately 50 minutes to complete.After a short break participants did their second set, which

    took about 40 minutes on average.

    III. RESULTS

    A. Expert validity

    To verify the agreement between the experimenters sort-

    ing and those by the three experts, we recoded the sorting

    data into a list of problem pairs. Each pair could have one of

    two values: together or apart (see the Appendix). With

    the data in this format, we could compute an inter-rater

    reliability. Because we had four raters, we used an intraclass

    correlation coefficient [53]. Values were R 0:77 for theelectricity problems andR 0:80for the magnetism prob-lems (two-way random effects, average value). Finally, with

    respect to the number of stacks, the external experts created

    about six stacks on average (electricity: M 5:67,SD 1:15; magnetism: M 6:33, SD 0:58), while inthe design we had set on four clusters. Looking at the stack

    labels the experts gave (see the Appendix), it turns out that

    most of the labels could be matched to one of the experi-

    menters categories, but that some of the experts made

    further subdivisions according to geometry or mathematical

    technique. For instance, one expert distinguished between

    Amperes law with surface current and Amperes law with

    spatial current. These subdivisions did not reveal any con-

    sistent pattern across individuals, however.

    Taken together, these findings indicate that the com-

    bined judgments of experts and experimenters provide a

    reliable judgment of problem similarity. For most problem

    pairs there is agreement as to whether they belong

    together or not (see the Appendix for a full overview).

    For a few problem pairs, opinions vary. When it comes

    to judging the expert-likeness of students sortings,

    these ambiguous combinations will be left out ofconsideration.

    B. Nature of novices categorizations

    The next question is whether the novices problem sort-

    ings reveal the intended category structures. To answer this

    question, we considered the numbers of stacks, the clusters

    that emerged in a cluster analysis, and the types of stacknames students gave. In order to make the results directly

    comparable to those of the experts, only the nonelaborated

    sorting tasks were included in this analysis.

    TABLE V. Average numbers of stacks for all groups.

    Electricity Magnetism

    Experts (n 3)a M (SD) 5.67 (1.15) 6.33 (0.58)

    Proficient students (n 36) M (SD) 5.97 (1.50) 5.64 (1.51)Less proficient students (n 35) M (SD) 5.94 (1.73) 5.69 (1.49)

    aThe problem sets presented to the experts originally consisted of 23 problems (electricity) and22 problems (magnetism), respectively. For this table, in order to make figures comparableacross participant groups, only the problems that were kept in the final design have been takeninto account.

    TABLE IV. The experimental setup.

    Group A B C D

    n 20 20 20 20

    First set Electricity, with elaboration Electricity, no elaboration Magnetism, with elaboration Magnetism, no elaboration

    Second

    set

    Magnetism, no elaboration Magnetism, with elaboration Electricity, no elaboration Electricity, with elaboration

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    The students, like the experts, on average created more

    stacks than the four intended in the design (Table V).

    The number of stacks was not significantly different

    across participant groups (electricity: F2; 71 0:050,p 0:951; magnetism: F2; 71 0:303, p 0:739).The number of valid observations is slightly lower than

    the number of participants, because some students had

    handed in incomplete results, for instance, by omitting a

    problem number from their written results.To reveal patterns across participants, we used a hier-

    archical cluster analysis. Based on the sorting data, theprocedure first computes the dissimilarity, or distance,

    between each pair of problems. After that, the two prob-

    lems that are closest are taken together to form a cluster. In

    the next step the problems or clusters with the next smallest

    distances are taken together, and so on, until all problems

    are linked together. The analysis was performed using the

    statistical software package SPSS. As a distance measure

    we used Euclidean distance, which is the most com-

    monly used. As a linkage method, we used between

    groups average linkage as a robust multipurpose method

    [54]. The outcomes were interpreted using a dendrogram

    plot, in which the cluster structure is represented as a

    branching tree, with problems that were placed together

    more frequently being represented by a more closeconnection.

    Figures 1(a)1(d) present hierarchical cluster analysesfor electricity and for magnetism problems, both for more

    and less proficient students. For the proficient students,

    if we consider the topmost four clusters in both sets,

    the clusters are in line with the design, except for two

    problems in the electricity set (E14 and E17) and two in

    (a) (b)

    (c) (d)

    FIG. 1. (a) Cluster analysis, electricity, proficient students (n 19). (b) Cluster analysis, electricity, less proficient students(n 16). (c) Cluster analysis, magnetism, proficient students (n 17). (d) Cluster analysis, magnetism, less proficient students(n 19).

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    the magnetism set (B7 and B14). For the less proficient

    students, the appropriate number of clusters is less evident,

    and not all problem categories emerge from the analysis

    equally clearly. For the electricity problems, a five cluster

    interpretation yields three mismatched problems (E5, E17,

    and E18), with the Coulomb problems being dispersed

    over all clusters and the dipole category split in two. For

    the magnetism set, a four cluster interpretation yields two

    mismatched problems (B7 and B11) and a mix up of

    inductance and dipole problems. However, at a lower level

    in the tree, subgroups of inductance and dipole problem

    emerge as distinct clusters again. Thus, both for more and

    for less proficient novices, it turns out that in both sets most

    problems that were intended to be similar are linked to-

    gether more closely than problems that were intended to be

    different.

    In order to triangulate the outcomes of the cluster analy-

    sis, we looked into the names participants gave their prob-

    lem stacks (see Table VI for an example). This analysiswas conducted by two of the authors separately, after

    which differences were resolved by discussion. We distin-

    guished labels that indicated a solution method (frequent

    examples are: Gauss law, Gauss law in differential form,

    image charge, dipole approximation, Amperes law, Biot-

    Savarts law, sometimes with added specifications about

    symmetry, algebraic procedure. etc.); labels that only men-

    tioned objects or quantities (e.g., moving charge, field), a

    geometrical property (e.g., planar symmetry), or an alge-

    braic procedure (e.g., integration); and noncontent labels

    (e.g., insight, difficult, standard problem). In line with the

    outcomes of the cluster analysis, the majority of the labels

    indicated a solution method. On average the moreproficient participants had a higher proportion of their

    labels indicating solution methods (64%) than the less

    proficient participants (49%), F1; 78 7:3, p 0:008,2 0:086.

    C. Quantifying the similarity to expert sorting

    In order to quantify the gradual differences between

    novice and expert sortings, we had to express the (dis)

    similarity of an individual sorting to an expert sorting in a

    single number. To this end, the most objective measure is

    directly based on combinations of individual problems. If

    the experimenters and at least two of the experts had put a

    pair of problems together, the combination was judged

    expertlike. If neither the experimenters nor any of the

    experts had put the two problems together, the combination

    was judged expert-unlike. All other combinations, which

    were made by some of the experts but not by all, were

    neglected in computing the expert-likeness. Thus, for each

    participant we had two scores per sorting:

    Ni; like: number of expertlike combinations subjectimade,

    Ni; unlike: number of expert-unlike combinations subjectimade.

    For each set of cards we had two normalization

    parameters:

    Nmax-like: maximum number of expertlike combinations,

    Nmax-unlike: maximum number of expert-unlikecombinations.For the electricity problems Nmax; like 32 and

    Nmax; unlike 123, leaving 35 possible combinations to beneglected. For the magnetism set, Nmax; like 18,Nmax; unlike 131, and 41 to be neglected. The resultingexpert-likeness score E for subject i was calculated fromthe following formula:

    Ei Ni; likeNmax-like

    Ni; unlikeNmax-unlike

    :

    A perfect sorting would yield the maximum score,

    1, a random sorting should give an outcome close to

    zero. To test the scores sensitivity to random variations,

    and to compare the performance of the subjects to chance,

    we generated a set of 1000 random sortings. In these

    sortings the number of stacks per sorting was distributed

    binomially with the average number of clusters set to 5.9,

    which corresponds to the average number in the real data(cf. TableV). Both the scores for real participants and the

    artificially generated scores are presented in Table VII.

    TABLE VI. Stack labels by a more and a less proficient student.

    Electricity Magnetism

    More proficient Gauss Dipoles

    Image charges Rotations in magnetic field

    Dipoles Amperes circuit law

    Non-Gaussian field computations Biot-Savart integration

    Fluxes

    Less proficient Coulomb FluxDipole Sum field

    Capacitor Symmetry

    Poisson (integral) Magnetic moment

    Gauss Magnetic field

    Conductor

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    The data confirm that the random variations are small

    compared to the real scores for both the electricity and the

    magnetism cards, and that both groups of novices do con-

    siderably better than chance. To ensure that the assump-

    tions underlying the analysis of variance method (ANOVA)

    were met, we verified that neither the Kolmogorov

    Smirnov test of normality nor the Levene test for the

    homogeneity of variances indicated any significantdeviations.

    D. Effects of presenting elaborations

    We expected problem-sorting performance to depend on

    student level, on the presence of elaborations, and on the

    interaction between both factors. We had planned to assess

    the effects of elaborations within subjects through a

    repeated-measures ANOVA. Because the data (TableVII)

    suggest that the effects of elaborations might be different

    for electricity and magnetism problem sets, a third experi-

    mental factor, task version, was included in the analysis.

    This factor produced significant interactions, whichimplies that the two tasks cannot be regarded as parallel

    tasks. Therefore, we will provide separate analyses for the

    electricity and the magnetism problems.

    For the electricity problems, we found a significant

    positive main effect for student level, no significant main

    effect for the presence of elaborations, and a significant

    interaction between student level and the presence of elab-

    orations, with a medium effect size (Table VIII). Within-

    group analysis revealed a significant positive effect of

    elaborations on proficient students scores, F1; 67 11:6, p 0:001, and no significant effect on less proficientstudents scores,F1; 67 2:92, p 0:092.

    For the magnetism problems we found a significantpositive main effect for student level, a significant positive

    main effect for the presence of elaborations, with a me-

    dium effect size, and no interaction between student level

    and the presence of elaborations (TableIX).

    IV. DISCUSSION AND CONCLUSIONBoth the cluster analyses and the analysis of stack names

    confirmed that, for the domain of electricity and magne-

    tism, physics students already start to know solution types

    in the initial phases of their studies. Nevertheless, the

    expert similarity scores indicate that students quite often

    fail to identify a suitable solution approach. The expert-

    likeness scores also confirm that the proficient novices did

    significantly better than the less proficient novices.

    Our central hypothesis was that performance on the

    categorization task would improve if participants were

    supported to elaborate on the problem by getting a first

    elaboration to start with. For the magnetism problems, the

    expected main effect was confirmed. For the electricity

    problems, the difference was in the same direction,

    although the effect was not significant. Because the main

    effects were not significantly different across both tasks

    (t 1:28,p 0:2), we conclude, with some caution, thatthe hypothesis was confirmed.

    Our final hypothesis was that the gain in performance

    would be greater for the more proficient novices, based on

    the idea that, in order to be useful, elaborations need to be

    TABLE VIII. ANOVA with electricity problem-sorting score

    as dependent variable.

    df F p 2

    Proficiency (P) 1 22.10

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    integrated in a coherent problem representation. The hy-

    pothesis was confirmed for the electricity problems, where

    the proficient students performed better with elaboration

    than without, whereas for the less proficient students there

    was no such effect. However, for the magnetism problems

    the interaction effect was not significant and the trend was

    in the opposite direction. Findings with regard to the

    interaction effect were significantly different across both

    tasks (t 2:40, p 0:02). Taken together, in this studyproficient students performed significantly better with

    elaborations, regardless of task version, whereas for theless proficient students the evidence is inconclusive.

    As a limitation of the current study it should be noted

    that, although the problem-sorting method is suitable to

    establish the effects of given elaborations, it provides only

    little insight into the ways these elaborations affect the

    reasoning process, and why one elaboration might be

    more effective than another. Nevertheless, given the sig-

    nificant difference between both task versions, we turned to

    our stimulus materials again in search of an explanation.

    Upon close inspection we found that in the magnetism

    problem set some elaborations contained words that mighthave supported a keyword strategy (for instance, elabora-

    tions for the induction problems containing the word

    flux). In the electricity set, we had been more successful in

    avoiding such keywords. As a consequence, we speculate

    that the electricity elaborations would only be useful if they

    could be integrated in a coherent problem representation.

    An alternative explanation for the effect of giving elabo-

    rations might be that they do not so much provide new

    information, but rather stimulate a greater depth of pro-

    cessing. To test this hypothesis a follow-up study could

    provide placebo elaborations (e.g., repeating informa-

    tion already present in the problem) or distracting elabo-

    rations (suggestive of an unproductive solution approach).In this study both more and less proficient students

    turned out to know solution types, but they had difficulties

    matching problem situations to the right type. Furthermore,

    proficient students seem to induce a mental model of the

    situation that can be used to integrate elaborations, whereas

    less proficient students may only benefit if they can extract

    a keyword that is directly linked to a proper solution

    approach.

    Both the differences between our two tasks, and the

    differences between this study and some of the studies

    discussed in the Introduction, clearly illustrate how novi-

    ces performance strongly depends on domain and task

    format. It therefore remains to be seen how these findingstranslate into different domains, such as mechanics.

    Furthermore, because failure as well as successful per-formance can occur for many reasons, performance data

    alone are not sufficient to gain insight into students knowl-

    edge and reasoning processes. Further research is needed

    to address the ways more or less proficient novices elabo-

    rate on situational features in their problem representa-

    tions, and the ways this reasoning develops. This could

    be done, for instance, through think-aloud protocols and

    microgenetic approaches.

    With respect to educational practice, our findings

    suggest little reason to have instructional strategies

    targeting a fundamental restructuring of knowledge.Rather, instruction should be aimed at students under-

    standing of the situations and of the situational features

    that make a solution approach into a useful one. Promising

    approaches to this end are to present more context-rich

    and open-ended problems where a situational analysis

    cannot be avoided [55,56], to have the teacher act as a

    model demonstrating how particular elaborations can be

    helpful in the domain [57,58], and to have the students

    reflect on worked out solutions [25,59,60]. For many teach-

    ers, this could imply a shift in their selection of practice

    problems [61].

    APPENDIX: FULL OVERVIEW OF PROBLEM

    CARDS AND EXPERT SERVINGS

    See separate auxiliary material for full collection of

    problem cards and expert servings.

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